1. Field of the Invention
This invention relates generally to a corona igniter for emitting a radio frequency electric field to ionize a fuel-air mixture and provide a corona discharge, and a method of forming the igniter.
2. Related Art
Corona discharge ignition systems include an igniter with a central electrode charged to a high radio frequency voltage potential, creating a strong radio frequency electric field in a combustion chamber. The electric field causes a portion of a mixture of fuel and air in the combustion chamber to ionize and begin dielectric breakdown, facilitating combustion of the fuel-air mixture. The electric field is preferably controlled so that the fuel-air mixture maintains dielectric properties and corona discharge occurs, also referred to as non-thermal plasma. The ionized portion of the fuel-air mixture forms a flame front which then becomes self-sustaining and combusts the remaining portion of the fuel-air mixture. Preferably, the electric field is controlled so that the fuel-air mixture does not lose all dielectric properties, which would create a thermal plasma and an electric arc between the electrode and grounded cylinder walls, piston, or other portion of the igniter. An example of a corona discharge ignition system is disclosed in U.S. Pat. No. 6,883,507 to Freen.
The central electrode of the corona igniter is formed of an electrically conductive material for receiving the high radio frequency voltage and emitting the radio frequency electric field to ionize the fuel-air mixture and provide the corona discharge. The electrode typically includes a high voltage corona-enhancing electrode tip emitting the electrical field. The igniter also includes a shell formed of a metal material, and an insulator formed of an electrically insulating material disposed between the shell and the central electrode. The igniter of the corona discharge ignition system does not include any grounded electrode element intentionally placed in close proximity to a firing end of the central electrode. Rather, the ground is preferably provided by cylinder walls or a piston of the ignition system. An example of a corona igniter is disclosed in U.S. Patent Application Publication No. 2010/0083942 to Lykowski and Hampton.
During operation of high frequency corona igniters, there is an electrical advantage if the outer diameter of the insulator increases in a direction moving away from the grounded metal shell and towards the high voltage electrode tip. An example of this design is disclosed in U.S. Patent Application Publication No. 2012/0181916. For maximum benefit, it is often desirable to make the outer diameter of the insulator larger than the inner diameter of the grounded metal shell. This design has resulted in the need to assemble the igniter by inserting the insulator into the shell from the direction of the combustion chamber, referenced to as “reverse-assembly”. However, the reverse-assembly method leads to a range of operational and manufacturing compromises which may be unacceptable. For example, when disposing the assembly in an internal combustion engine, it is difficult to retain the insulator in the shell without putting the insulator in tension. Typically, the tension in the insulator increases once the assembly is installed in the engine.